Doctors are warning that the plummeting rate of heart attacks over the past 20 years, thanks to improvements in medication and a decline in smoking, is in danger of being reversed by a rise in obesity.

A survey of nearly 10,000 London civil servants taking part in the long-running Whitehall II study, which began in 1985, has shown a 74 per cent drop in the risk of a heart attack up to 2004. Better control of cholesterol, reduced blood pressure and the fall in smoking accounted for more than half of the decline.

But the researchers from University College London Medical School said the decline would have been even greater without the expansion in waistlines. Writing in the European Heart Journal, they estimated that the rise in body-mass index over the 20 years has increased the risk of a heart attack by 11 per cent.

Sarah Hardoon, who led the study, said: "The substantial decline... highlights what can be achieved. Although these favourable trends seem to have outweighed the negative contribution of rising BMI over recent decades, continued increases in BMI may reduce further, and even reverse, the decline in the incidence of heart attacks in the future." Obesity has almost doubled in the past 20 years, with nearly one in four adults classed as obese (BMI 30 or over) in 2005. In younger people, there are already signs that the decline in deaths from heart disease is flattening out and could again start rising.

The study found that medication played a greater role than improvements in diet or exercise in bringing down the heart-attack rate. Statins for lowering cholesterol were taken by more than one in 10 of the civil servants by the end of the study. Dr Hardoon's earlier research showed that drugs were responsible for two-thirds of the reduction in blood pressure – and changes in lifestyle responsible for only a third.

Fruit and vegetable consumption increased over the period but made little difference to the outcome. Trends in physical activity and alcohol consumption had no notable impact.

In total, these factors accounted for just over half (56 per cent) of the decline. Speculation continues about what explains the rest – one theory is that there may be a viral cause of heart disease, implying that it can be caught like an infection.

"There is a lot of debate about whether there are other novel risk factors out there," Dr Hardoon said.

Coronary heart disease affects the "crown" or corona of arteries which sit on top of the heart and provide oxygenated blood to the heart muscle to keep it pumping. When these small arteries are blocked by a blood clot, it causes a coronary thrombosis, or heart attack.

... but if you want to live longer, move to Surrey

Women from Waverley, in Surrey, are the most likely to live a long life, with an 86 per cent chance of making it to the age of 75, according to figures released by the Office for National Statistics (ONS) yesterday.

The data also showed that Northerners are much less likely to live to an old age than Southerners. Manchester residents had the lowest life expectancy in the country between 2007 and 2009, with men having only a 54 per cent chance of surviving to their mid-70s and women a 69 per cent chance. Men in South Buckinghamshire had the best hope of long life. Life expectancy has risen across England and Wales as a whole since 2003-05, when men had a 65 per cent chance and women a 77 per cent chance of living to 75. By 2007-09 these figures had risen to 68 per cent and 79 per cent respectively.

Regional variations narrowed but remained significant, with men and women in the North-east and North-west facing lower chances of survival.